Hot gas defrost
Also known as: reverse cycle defrost, hot gas defrosting
Short definitionMelting the ice on an evaporator by passing hot vapour from the compressor through the inside of its tubes. It is quicker than heater defrost and uses less energy, but the pipework is busier.
In a quote for a large frozen store or a blast tunnel, this is usually the method written on the defrost line. Vapour taken from the discharge line is at 60–80 °C; valves send it to the evaporator, the ice warms from inside and runs off in 5–10 minutes. No heaters are needed; what is used is heat the system would reject anyway. Because every part of the fins warms at the same time, the ice leaves no residue and the room warms less. It is the usual solution in central systems and in CO₂ and ammonia plants.
The cost is in the installation. A separate pipe is run between the discharge line and the evaporator, solenoid or three-way valves are fitted, a bypass is made around the expansion valve and the return of the condensed refrigerant is sorted out. The evaporator is also ordered with a coil circuit made for this job. In small single-evaporator units the condenser and the evaporator swap duties; this is called reverse cycle. In a small room the expense does not pay back, and electric defrost is chosen.